They said the six deaths appeared to have occurred about 12 to 24 hours before police arrived on the scene.
The bodies showed traces of the rapid-acting chemical and had purple lips, indicating a lack of oxygen, Chulalongkorn Hospital's Kornkiat Vongpaisarnsin told reporters, adding more tests were being conducted.
At a press conference held earlier the same day, Thai police also said cyanide was found in all coffee cups at the scene, and one of the six is believed to have poisoned the others.
It was confirmed that only the six entered and left Room 502 of the hotel, according to Pol Maj Gen Noppasilp Poolsawat, deputy commissioner of Metropolitan Police Bureau.
Among the six, who travelled to Thailand from July 4-12, is a couple from Ho Chi Minh City, but they did not check into the hotel at the same time.
Noppasin said the case appeared to be personal and would not impact the safety of tourists.
Also at the press conference, police said debt issues are the motive behind the crime, and that the seventh person who was part of the hotel booking left Thailand for Vietnam on July 10.
